1. Field of the Inventions
The present inventions relate to exhaust systems for outboard motors, and more particularly, idle exhaust systems for outboard motors.
2. Description of the Related Art
Examples of conventional outboard motors include one disclosed in Japanese Patent Document JP-A-2000-303818. An outboard motor in this patent document includes a case extending longitudinally that can be supported on the hull of a boat and whose lower part can be immersed in water. A propeller is supported on the lower end portion of the case. Additionally, an engine is supported on the upper surface side of the case and operatively coupled with the propeller.
In this design, inside the case, there are formed a main exhaust passage for introducing exhaust produced during operation of the engine to the inside of the case and discharging the exhaust from the inside of the case into water, and an idle exhaust passage that branches off from the main exhaust passage to discharge a part of the exhaust to the atmosphere above the water line. Further, an expansion chamber is formed at a midway portion of the idle exhaust passage. The downstream end of the idle exhaust passage serves as an idle exhaust port that is open to the atmosphere.
When the propeller is operated in synchronization with this engine during operation of the engine, this allows the boat to be propelled. Most of the exhaust produced during the operation of the engine is discharged into the water by way of the main exhaust passage below the waterline. This discharge of the exhaust into the water prevents generation of exhaust noise.
On the other hand, it is difficult to discharge exhaust produced during idling of the engine into the water by way of the main exhaust passage due to its low exhaust pressure. In other words, the water pressure prevents, even at a position immediately below the surface of the water, can prevent exhaust gas from smoothly being discharged during idle operation. Accordingly, the exhaust produced during idling is discharged to the atmosphere by way of the idle exhaust passage above the waterline. In this case, the noise of this exhaust is muffled as the exhaust passes through the expansion chamber at a midway portion of the idle exhaust passage. The generation of exhaust noise during idling is thus also prevented.